1. Field of the Invention
The present invention generally relates to methods for shaping the corners of deep trenches in a semiconductor transistor to increase the transistor""s useful life.
1. Description of the Related Art
Conventional technologies utilize metal oxide semiconductor field effect transistors (xe2x80x9cMOSFETxe2x80x9d) bounded by deep and shallow trenches for storage nodes and device isolation, respectively. FIG. 17 illustrates a conventional deep trench bounded MOSFET device which includes polysilicon gates 172 which wrap around the edge of the device. These trenches conventionally include an abrupt corner 173 and a gate wraparound 174 at the edge of the active area, which cause the device to be characterized by one threshold voltage for the corner and another corresponding to the channel.
Ideally, the threshold voltage of the corner device should be equal to the threshold voltage of the channel Vt. In addition, abrupt geometry at the corner of a MOSFET leads to higher electric fields and higher Fowler-Nordheim tunneling current through the gate oxide at the corner than in the channel region.
Fowler-Nordheim tunneling current is due to the wave nature of an electron which allows it to xe2x80x9ctunnelxe2x80x9d from the valence band of the p-type semiconductor when it approaches the forbidden gap and appear at the same energy in the conduction band of the n-type semiconductor. The probability of this occurring is a strong function of the thickness of the barrier.
Such currents increase the risk of dielectric wearout at the deep trench corner 171 and deep trench/STI (shallow trench isolation region) intersection 173, 175. Control of the shape of the deep trench corner, then, is an important factor influencing both the corner voltage threshold Vt and the gate oxide wear at the edge of the device.
The conventional structure is also shown as an equivalent planar device in FIG. 18 and includes corners 180, a channel 181 and an oxide 182 which has a thickness of approximately 100 xc3x85. Due to field enhancement at the non-rounded deep trench corners, the effective thickness of the gate oxide is reduced at these points.
The degree of effective oxide thinning at the corner is determined by the sharpness of the corner, a factor which can be quantified by the ratio of the radius of curvature of the corner relative to the thickness of the overlying gate oxide (R/Tox) at the corner. FIG. 19 illustrates the effective oxide thinning at the corner graphically. FIG. 19 was prepared assuming a deep trench corner having a radius of curvature 1.5xc3x97 the thickness of the gate oxide and an infinite conducting plate separated from a conducting cylinder by a distance equal to the thickness of the gate oxide (e.g., 100 xc3x85). The field enhancement illustrates the effective thickness of the gate oxide at the deep trench corner relative to that in the planar channel region of the device.
The enhanced field across the gate oxide at the deep trench corner decreases the corner Vt relative to the channel which contributes to a sub Vt leakage mechanism. This enhanced field will also degrade the reliability of the gate oxide at the corner. The electric field at the edge of the deep trench ultimately controls both the threshold voltage of the corner device and the reliability/wearout of the gate oxide. The ability to affect the shape (e.g., the radius of curvature) of the deep trench corner is critical in controlling these characteristics.
However, conventional processes cannot control the shape of the trench corner with precision. Therefore, the corners of the conventional trench devices will generally be very sharp, have a reduced oxide thickness, and exhibit reduced threshold voltage and excessive wear characteristics.
It is, therefore, an object of the present invention to provide a structure and method for increasing the threshold voltage of the corner device on a trench bounded metal oxide semiconductor field effect transistor (MOSFET) structure by controlling the shape of the corner. More specifically, the invention rounds the corners of the deep or shallow trenches which bound a MOSFET device. With the invention, the degree of field enhancement in regions of abrupt geometry (e.g., the corners) can be minimized and fine tuned.
A key advantage of the invention is that the amount of corner rounding is precisely controlled, which enables the corner device threshold voltage to be xe2x80x9ctunedxe2x80x9d to a desired value. For example, it would be preferable to increase the threshold voltage of the corner device to match the threshold voltage of the channel. As discussed in greater detail below, the invention uses wet etching to remove the oxide film under the pad nitride layer, thereby forming a recess which is subsequently filled with nitride from the node dielectric film. The resulting nitride plug controls the extent to which the trench corner is oxidized in subsequent processing steps.
The invention also controls the shape of the corner by controlling the stripping time of the passivation film so as to recess the pad the oxide film. By controlling the degree to which the pad oxide is recessed, the thickness of the pad oxide film and the amount of nitride remaining in the recess, the rounding of the trench corner can be controlled.
Thus, with the invention, the electric field enhancement and the threshold voltage at the corner of the device can be tuned to a desired value. The invention includes methods of shaping the edge of the deep trench in order to raise the corner threshold voltage to better match the channel threshold voltage Vt.
Several thermal oxidation steps determine the shape of the deep trench corner as well as the thickness of the gate oxide at the corner. These steps include the formation of the deep trench sacrificial oxide, the collar passivation oxide, the gate sacrificial oxides and the gate oxide. During the deep trench sacrificial oxide and collar preservation oxide processes, initial corner shaping occurs through the formation of a bird""s beak under the pad films. While the final corner shape of the gate oxide thickness around this edge is determined by the gate sacrificial oxide and gate oxidation processes, the formation of a bird""s beak during the deep trench related processes provides a sensitive means for controlling the ultimate shape of the deep trench corner.
The invention has the added benefit that it also controls the reliability of the gate oxide by influencing the Fowler-Nordheim tunneling current through the gate oxide at the edge of the device.